Epic Test Enhancements

## Reason

This epic incorporates the following kind of issues

- Improvement of existing tests to increase test 'depth'
- Addition of unit tests to increase code coverage
- Addition of integration tests
- Separation of tests (i.e. integration tests, short tests etc.) (:exclamation: this should be a priority)
- Addition of new kind of tests such as fuzzing or property-based testing
- ~~Finding and fixing data races (:exclamation: this should be a priority)~~ (see #2147)
- Test tooling improvement (i.e. mock generation, CI)